>> They have in a split cage for almost 4 weeks. I tried an intro last week. They ended up in a gerbil ball. No blood drawn. <<
I posed about Stella dying a few weeks ago, and why Francis, her sister, will stay alone even though Stella died way too young (maybe a year), and I have no idea why, as I also posted.
I have tried hard many times with split screening: making the screen at an art supply store from old frames, and keeping the gerbs in there for 2-3 weeks, which I did not like as space was limited foer them. Well, I also posted on the results, which were usually not good.
>> I have never had a split cage go this long. <<
I never heard of it that long.
I never liked scent masking as once the scent wears off there can be fights.
Someone once told me that it helps to, a little before intros, or maybe longer than "a little" (I'm not sure) to rub each other's nose in the other's butt.
Of course perhaps neutral territory might help rather than just pulling the screen out of a tank they both were using.
Sadly, I usually just ended up with not only loads of work I have little time for, despite my efforts, TWO gerbils living alone. Plus, I would never take one gerbil from a pair from a breeder leaving the other alone.
I've had some single rescues, and I tried those, but even then I had problems. I posted about the rescued Hermione: she went through a few tank mates who, after tons of effort and worry by me, accepted her, But, the newbies died after several months! No idea why. Finally, I did get her one she not only accepted but lived with for a long time until she died too; by then Hermione was old and I did not try again.
Right now, after some deaths, I have one pair and one gerbil, a total of three. Different genders, too. So, no screening for them. The single gerb, Francis, is in a tank closest to me. We do what we can. As my thread in the Miscellany forum shows, I have also been awfully busy with computer screwups.
>> Would doing brief intros at night for a few minutes help? <<
I posted some years ago about the "Gang of Four". You can Seach for their story. Some of my first gerbs. Tommy was one of my favorite critters ever, smart and friendly. One day I sadly found him dead.
In the trio of remaining gerbs, his brothers soon fought. I was not sure what I heard at first but a day later I heard what was surely a horrendous fight. Dill and Phil were both badly mauled and their photos should be still up in this forum; easy to find. Cliffy stayed out of it. Only one survived and he lived happily with Cliffy.
Their photos are in my Memorial Gallery in my signature.
My point is that once a fight starts SEPARATE AT ONCE AND FOREVER. Gerbils will not make up and get friendly. I should have separated Phil from Dill at once, but again, I was not sure what happened.
I fear that once gerbs ball up there are no successful retries. If they do ball up I would certainly move them away from each other for a few weeks and only then try the split screen again. But I would have little hope of it working.
I have often seen gerbs sleep next to each other against the mesh, only to go wild once it was pulled and try to kill each other. One of my dead gerbs, Phil (above story) took only one bite in the abdomen and it killed him in two days. Dill looked awful and half skinned alive, but he survived! Just one bite can be fatal. Find the old thread and see the photos.
Your situation is yours and I only speak in generalities. But consider all this a caveat.
Good luck.